Unmatched cab/top ohm. Safe or unsafe?

[JonatanOTG]

Maybe someone in this forum has some wisdom to share on the matter?

Simply put, I wonder if I can, safely, connect and use a 4 ohm cab with a 8 ohm amp? It's a combo but the the internal speaker is muted when you connect an external.

After doing some research on the subject, I decided that this is one of the few guitar-related things you are not supposed to try out on a trial-and-error basis.

I have literally heard people say that one can die if you connect the wrong cab to the wrong top. So I went to a music store and asked them, just to be sure. He told me that yes, if you connect a cab with more ohm than the top you are risking your life. Then he added, after a moment of thought: It might be the other way around though, less ohm than the top, I'm honestly not sure. Well. That didn't feel to ensuring!  :doh:

So I figured I could ask you guys instead  :icon_thumright:

 

[stratamania]

Generally not recommended. You could damage the amp.

Some of the other guys on the forum have built amps so could probably explain the science.

 

[Cagey]

I don't know why anybody would say you could die from it, unless they were just kidding.

In any event, don't do it. Generally speaking, you won't hurt anything immediately, but eventually the amp will fail. The timing changes based on amp design - solid state amps are fairly easy to kill and will often die quickly. Tube-based units take longer, but they'll tilt, too. Besides, even if it didn't wreck the amp, it wouldn't sound right.

 

[pabloman]

How many speakers are in the extension cab? If there is more than one what are the speakers rated at? What model is the amp and cabinet? You may be able to rewire them for an 8 ohm load.

 

[line6man]

8 Ohms parallel to 4 Ohms is 2.67 Ohms. If your amp can handle a 2.67 Ohm load, then there is no problem on the amp side. Take note, however, that the distribution of power to the speakers will be uneven. You need to be sure that each speaker can handle what it is receiving from the amp, otherwise there will be damage.

 

[mayfly]

Is this a tube amp?  If so, you need to match the cabinet and the head. 

If the cab has a higher impedance than the amp (not your case), the amp will produce less power and the rig will sound funny.

If the cab has a lower impedance than the amp (your case), the amp will try to produce more power at the expense of plate dissipation.  That is, it will drive the output tubes beyond their maximum rating.  This will eventually blow something up.  I've seen a pair of marshalls where someone did this at high volume for a while (a gig using someone else's cabinets).  They both blew up at the show.  What happened to each amp is a plate failed in an output tube, which shorted the plate to ground.  This shorted the output transformer to ground on the plate end.  The other end is tied to the B+ supply (over 400V for these old marshalls) with no fuse in between.  boom.  Very expensive repair.

 

[mayfly]

How many speakers are in the extension cab? If there is more than one what are the speakers rated at? What model is the amp and cabinet? You may be able to rewire them for an 8 ohm load.

This might be your solution actually.  Got any pictures for us?